The invention relates to a ridge waveguide-type optical semiconductor device and to a method of fabricating the same.
Optical semiconductor devices which employ an optical waveguide layer (core layer) between clad layers, for example a semiconductor laser diode or an optical electro-absorption modulator, are known. In general, these optical devices require a single parallel horizontal mode. A ridge waveguide is preferred to provide the single transverse mode.
In manufacturing a ridge waveguide-type optical semiconductor on a semiconductor substrate, a first clad layer, an optical waveguide layer (core layer), a second clad layer and a contact layer are stacked sequentially. The stacked structure is etched selectively so that a belt-like ridge portion consisting of the second clad layer and the contact layer is formed.
In forming the ridge, the etching does not completely remove the second clad layer, so that the remainder of the second clad layer forms a mode control layer at both side of the ridge. The mode control layer favors a single transverse mode. However, controlling the thickness of the mode control layer is difficult. Therefore, it is difficult to consistently manufacture the device with the same properties.
It is therefore an object of the invention to provide a ridge waveguide-type optical semiconductor device and a method of fabricating the same, which solve the above-described problem. According to the present invention, such a method includes forming a first clad layer, a core layer to serve as a waveguide, a second clad layer and a contact layer, in the stated order on a semiconductor substrate, forming a strip-shaped etching mask on the contact layer, removing the contact layer and the second clad layer selectively by etching, using the etching mask, until the core layer is exposed, so as to form a ridge including the etched second clad layer, and forming a semiconductor mode control layer by crystal growth, on the exposed surface of the core layer.
Therefore, contrary to the above-described conventional method, the mode control layer is formed only after the second clad layer is etched selectively. Compared to the conventional etching process, it is possible to form the mode control layer with a required thickness and showing comparatively greater uniformity. As a result, it is possible to fabricate the ridge waveguide-type optical device consistently of with the same mode properties.
Moreover, according to the invention, the refractive index of the mode control layer is selected so as to confine light within a waveguide. Also, the semiconductor material for the mode control layer is selected in view of the desired electrical operation of the device. Therefore, the efficiency of operation of the device is improved.